Robust unicast/broadcast/multicast communication protocol

ABSTRACT

Methods and apparatus for implementing a robust unicast/broadcast/multicast protocol are provided. In one aspect, a method of avoiding collision of intra-basic service set unicast, broadcast or multicast transmissions notifies stations in the basic service set of a reserved transmit opportunity for a unicast, broadcast or multicast transmission. Transmissions from at least one station in the basic service set are deferred until after the reserved unicast, broadcast or multicast transmit opportunity.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. patentapplication Ser. No. 12/494,122, filed Jun. 29, 2009, which claims thebenefit under 35 U.S.C. §119(e) of U.S. Provisional Patent ApplicationSer. No. 61/142,146, filed Dec. 31, 2008, entitled “RobustBroadcast/Multicast Communication Protocol”. Each of these references ishereby incorporated by reference in its entirety.

BACKGROUND

1. Technical Field

This disclosure generally relates to the field of transmission of audioand video streams in wireless local area network (WLAN) communicationsand, more particularly, to quality of service for multicast andbroadcast of audio/video streams in WLAN communications.

2. Description of the Related Art

In recent years, wireless computer networks such as WLANs have beenwidely deployed in places ranging from homes and businesses to hotels,airports, schools, etc. Compared to wire networks, a WLAN provides usersmobile connectivity and are typically more scalable and require lowercost in network construction. With the popularity of multimediaapplications like streaming multimedia and voice over IP on the rise, itis important to ensure quality of service in the delivery oftime-sensitive multimedia content in a WLAN.

Within the IEEE 802.11 WLAN standards, two access protocols at the MAClayer are defined: the mandatory distributed coordination function (DCF)and the optional point coordination function (PCF). The DCF protocoluses a carrier sense multiple access with collision avoidance (CSMA/CA)method to decide which station in the WLAN should send packets out. TheWLAN stations listen to the wireless medium to determine when it isfree, or idle. Once a station detects that the medium is idle, thestation begins to decrement its back-off counter. Each station maintainsa contention window (CW) that is used to determine the number of slottimes a station has to wait before transmission. The back-off counteronly begins to decrement after the medium has been idle for a DCFinter-frame space (DIFS) period. If the back-off counter expires and themedium is still idle, the station begins to transmit. Thus, it ispossible that two stations may begin to transmit at the same time, inwhich case a collision occurs. If a frame is successfully received bythe destination the frame is addressed to, the destination sends anacknowledgement (ACK) frame to notify the source of the successfulreception. However, as there is no centralized coordination in DCF, arelatively high number of collisions still occur especially in a WLANwith high load.

In an infrastructure basic service set (BSS) with a WLAN, typicallyincluding an access point (AP) and associated non-AP stations (STAB),centralized coordination can be achieved when using the PCF protocol asPCF requires the AP to poll the stations to coordinate access to thewireless medium. However, PCF is rarely implemented as theimplementation is complicated because AP has to poll the non-AP stationsfor the transmission of each polled packet. Additionally, it isdifficult to integrate an effective power saving mode with the PCFprotocol.

The IEEE 802.11e is an approved amendment that defines a set of qualityof service enhancements for WLAN applications through modification tothe media access control (MAC) layer. Specifically, the IEEE 802.11eenhances the DCF and PCF protocols through the hybrid coordinationfunction (HCF). The HCF includes two protocols of channel access:enhanced distributed channel access (EDCA) and HCF controlled channelaccess (HCCA). As the HCCA protocol uses contention free periods totransmit data streams, it makes efficient use of the bandwidth. However,implementation of the HCCA protocol tends to require a relativelycomplex scheduler and added complexity. With the EDCA protocol, datastreams are assigned different priorities and thus high-priority datastreams have a higher chance of being transmitted than low-priority datastreams. Each priority level is assigned a respective TransmitOpportunity (TXOP), which is a time interval during which a station in aBSS can transmit as many frames as possible.

Video streams require reliable transmission and deterministic delay. Atcurrent time it is difficult, however, to provide reliable videotransmission in an IEEE 802.11-based WLAN because of insufficientsupport of broadcast and multicast transmissions. For example, anintra-BSS collision occurs if associated stations transmit frames whenthe AP is transmitting broadcast/multicast frames. When two or moreBSS's overlap each other, an overlapping BSS (OBSS) or inter-BSScollision might occur. The collision may happen, for example, if astation or the AP of a first BSS transmits frames when an AP of a secondBSS that overlaps the first BSS is transmitting broadcast/multicastframes. Furthermore, there is no feedback mechanism to indicate thecorrect reception of the broadcast/multicast frames transmitted by theAP.

In addition, the EDCA protocol currently does not provide deterministicdelay guarantee. Although there are efforts made to include areservation-based medium access method in the IEEE 802.11n and 802.11sdrafts, a number of issues still need to be addressed in order toprovide reliable reserved medium access for transmission of videostreams. For example, issues related to coexistence with legacy OBSS andcoexistence with or without OBSS should be considered.

BRIEF SUMMARY

A method of avoiding collision of intra-basic service set unicast,broadcast or multicast transmissions may be summarized as includingnotifying stations in the basic service set of a transmit opportunityfor a reserved time interval for a unicast, broadcast or multicasttransmission; and deferring transmission from at least one station inthe basic service set until after the reserved time interval. In oneembodiment, notifying the stations in the basic service set of thetransmit opportunity may include broadcasting an information elementrelated to the transmit opportunity in at least one beacon or actionframe to the stations in the basic service set. In another embodiment,broadcasting an information element related to the transmit opportunitymay include broadcasting at least one of the following: an activeindication to indicate whether or not the transmit opportunity iscurrently enabled, an activation start time to indicate a time when thetransmit opportunity will be enabled if the transmit opportunity is notcurrently enabled, a transmit opportunity type, a source address, adestination address, a service interval, an offset to the serviceinterval, and a duration of the transmit opportunity. In one embodiment,notifying the stations in the basic service set of the transmitopportunity may include broadcasting the transmit opportunity using adelivery traffic indication message (DTIM) beacon to stations in thebasic service set. In another embodiment, deferring transmission from atleast one station in the basic service set until after the reserved timeinterval may include setting a network allocation vector (NAV) in the atleast one station in the basic service set to stop frame transmissionduring the reserved time interval.

A method of avoiding collision of inter-basic service set unicast,broadcast or multicast transmissions may be summarized as includingreserving a first unicast, broadcast or multicast transmit opportunityin a first basic service set; reserving a second unicast, broadcast ormulticast transmit opportunity in a second basic service set thatoverlaps the first basic service set; exchanging information related tothe respective reserved transmit opportunity between the first and thesecond basic service sets; determining a difference of timingsynchronization function (TSF) values between the first and the secondbasic service sets; determining if an overlap between the reservedtransmit opportunities of the first and the second basic service setsexists based on the difference between the TSF values; and contendingfor medium access right using the EDCA protocol during the respectivereserved transmit opportunity in at least one of the first and thesecond basic service sets. In one embodiment, exchanging informationrelated to the respective reserved transmit opportunity between thefirst and the second basic service sets may include exchanginginformation related to the respective reserved transmit opportunitybetween the first and the second basic service sets through relayedaction management frames using stations of the first and the secondbasic service sets. In one embodiment, the method may further includerecording at an access point of at least one of the first and the secondbasic service sets the difference of the TSF values between the firstand the second basic service sets to calculate an overlap between thereserved transmit opportunities of the first and the second basicservice sets. In another embodiment, the method may further includeselecting in the first basic service set a new unicast, broadcast ormulticast transmit opportunity not overlapping the reserved secondtransmit opportunity of the second basic service set when the reservedfirst transmit opportunity of the first basic service set overlaps thereserved second transmit opportunity of the second basic service set.

A method of avoiding collision of inter-basic service set unicast,broadcast or multicast transmissions may be summarized as includingreserving a first transmit opportunity in a first basic service set;reserving a second transmit opportunity in a second basic service setthat overlaps the first basic service set, the second transmitopportunity overlapping the first transmit opportunity; randomlyselecting a respective priority number for each of the first and thesecond basic service sets to provide one of the first and the secondbasic service sets with a higher priority number and the other with alower priority number when the second transmit opportunity overlaps thefirst transmit opportunity at least partially; and releasing thereserved transmit opportunity by the basic service set having the lowerpriority number. In one embodiment, the method may further includeselecting a new transmit opportunity for the one of the first and thesecond basic service sets having the lower priority number, the newtransmit opportunity for the basic service set having the lower prioritynumber not overlapping the transmit opportunity reserved in the basicservice set having the higher priority number.

A method of avoiding collision of inter-basic service set unicast,broadcast or multicast transmissions may be summarized as includingdetecting an overlap between a transmission range of a first basicservice set and a transmission range of a second basic service set; andrespectively selecting at least one of a service interval, an offset,and a duration based on a default beacon interval or a default DTIMbeacon interval in reserving a transmit opportunity for a unicast,broadcast or multicast transmission in each of the first and the secondbasic service sets. In one embodiment, the first basic service set mayhave a first beacon interval and the second basic service set may have asecond beacon interval where the first beacon interval and the secondbeacon interval are different. In one embodiment, respectively selectingat least one of a service interval, an offset, and a duration based onthe default beacon interval or the default DTIM beacon interval inreserving a transmit opportunity for a unicast, broadcast or multicasttransmission in each of the first and the second basic service sets mayinclude reserving a first transmit opportunity in the first basicservice set based on the default beacon interval or the default DTIMbeacon; and reserving a second transmit opportunity in the second basicservice set based on the default beacon interval or the default DTIMbeacon, the first transmit opportunity and the second transmitopportunity not overlapping. In one embodiment, the method may furtherinclude indicating in an information element of the reserved transmitopportunity a difference between a first beacon interval of the firstbasic set and the default beacon interval. In another embodiment, themethod may further include adjusting a reserved service interval if thereserved service interval covers a first beacon interval of the firstbasic service set before the adjusting.

A circuit in a first wireless communication device of a first basicservice set may be summarized as including communication logicconfigured to generate a transmit opportunity information element toreserve a transmit opportunity for a unicast, broadcast or multicasttransmission, the communication logic further configured to ceasetransmission by the first wireless communication device during atransmit opportunity reserved by another wireless communication device.In one embodiment, the communication logic may be further configured tocontend for medium access rights using the EDCA protocol during thereserved transmit opportunity. In another embodiment, the communicationlogic may be further configured to randomly select a priority number ifthe reserved transmit opportunity overlaps a reserved transmitopportunity of a second basic service set, to release the reservedtransmit opportunity if the randomly selected priority number is lowerthan a randomly selected priority number for the second basic serviceset, and to select a new transmit opportunity not overlapping thereserved transmit opportunity of the second basic service set. In yetanother embodiment, the communication logic may be further configured touse a default beacon interval or a default delivery traffic indicationmessage (DTIM) beacon interval to select at least one of a serviceinterval, an offset, and a duration in reserving a transmit opportunityfor the unicast, broadcast or multicast transmission.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIGS. 1A and 1B are each a simplified diagram of intra-basic service setcollision or inter-BSS collision using conventional broadcast or unicastmethods.

FIG. 2 is a simplified diagram of a reserved video TXOP informationelement according to one embodiment.

FIG. 3 is a simplified diagram of transmission of video frames duringreserved video TXOP according to one embodiment.

FIG. 4 is a simplified diagram of time spots for broadcast of a reservedvideo TXOP information element according to one embodiment.

FIG. 5 is a simplified diagram of collision avoidance for inter-basicservice set broadcast or multicast transmissions according to oneembodiment.

FIG. 6 is a simplified diagram of collision avoidance for inter-basicservice set broadcast or multicast transmissions according to anotherembodiment.

FIG. 7A is a simplified diagram of collisions during inter-basic serviceset broadcast or multicast transmissions.

FIG. 7B is a simplified diagram of collision avoidance for inter-basicservice set broadcast or multicast transmissions according to yetanother embodiment.

FIG. 8A is a simplified diagram of inter-basic service set interferenceand collision avoidance when a station receives reserved TXOPinformation element from an overlapping basic service set's access pointaccording to one embodiment.

FIG. 8B is a simplified diagram of inter-basic service set interferenceand collision avoidance when an access point receives reserved TXOPinformation element from an overlapping basic service set's stationaccording to one embodiment.

FIG. 8C is a simplified diagram of inter-basic service set interferenceand collision avoidance when an access point receives reserved TXOPinformation element from an overlapping basic service set's access pointaccording to one embodiment.

FIG. 8D is a simplified diagram of inter-basic service set interferenceand collision avoidance when a station receives reserved TXOPinformation element from an overlapping basic service set's stationaccording to one embodiment.

FIG. 9 is a flow chart of a process for avoiding collision ofintra-basic service set broadcast or multicast transmissions accordingto one embodiment.

FIG. 10A is a flow chart of a process for avoiding collision ofinter-basic service set broadcast or multicast transmissions accordingto one embodiment.

FIG. 10B is a flow chart of a process for avoiding collision ofinter-basic service set broadcast or multicast transmissions accordingto another embodiment.

FIG. 11 is a flow chart of a process for avoiding collision ofinter-basic service set broadcast or multicast transmissions accordingto another embodiment.

FIG. 12A is a flow chart of a process for avoiding collision ofinter-basic service set broadcast or multicast transmissions accordingto still another embodiment.

FIG. 12B is a flow chart of a process for avoiding collision ofinter-basic service set broadcast or multicast transmissions accordingto yet another embodiment.

FIG. 13 is a simplified block diagram of a wireless communication devicein a WLAN communication system according to one embodiment.

DETAILED DESCRIPTION

As used herein, the term “broadcast” generally refers to transmission ofwireless signals to one or more unspecified devices capable of receivingwireless signals. The term “broadcast” may further refer to wirelesstransmission of data, audio, and/or video packets from an access pointof a basic service set to all stations of the basic service set in thecontext of a local area network such as a WLAN.

As used herein, the term “multicast” generally refers to transmission ofwireless signals to a plurality of specified destinations or devicescapable of receiving wireless signals. The term “multicast” may furtherrefer to wireless transmission of data, audio, and/or video packets froman access point of a basic service set to a subset of the stations ofthe basic service set in the context of a local area network such as aWLAN.

As used herein, the term “unicast” generally refers to transmission ofwireless signals to a specified destination or device capable ofreceiving wireless signals. The term “unicast” may further refer towireless transmission of data, audio, and/or video packets from anaccess point of a basic service set to one of the stations of the basicservice set in the context of a local area network such as a WLAN. Theterm “unicast” may also further refer to wireless transmission of data,audio, and/or video packets from one of the stations of a basic serviceset to one of the stations of the same basic service set in the contextof a local area network such as a WLAN.

As used herein, the term “transmission” generally refers to transmissionof wireless signals to any number of devices capable of receivingwireless signals via broadcast, multicast or unicast. The term“transmission” may further refer to wireless transmission of data,audio, and/or video packets from an access point or a station of a basicservice set to any number of the stations and access points in a basicservice set and any overlapping basic service set in the context of alocal area network such as a WLAN.

FIG. 1 illustrates intra-basic service set collision and inter-basicservice set collision using conventional unicast, broadcast or multicastmethods. An intra-basic service set collision can occur when one or morestations in the basic service set transmits frames when the access pointis transmitting unicast, broadcast or multicast frames to some or allthe stations in the basic service set. As shown in FIG. 1A, for example,an intra-basic service set collision occurs when station STA3 transmitframes while access point AP is transmitting unicast, broadcast ormulticast frames to some or all the stations in the basic service setBSS-A. An inter-basic service set collision can occur when an accesspoint transmits unicast, broadcast or multicast frames while one or morestations, or the access point, of an overlapping basic service settransmit frames. As shown in FIG. 1B, for example, an inter-basicservice set collision (or overlapping basic service set collision)occurs when the access point AP1 of the basic service set BSS-Atransmits unicast, broadcast or multicast frames while the access pointAP2 of the basic service set BSS-B is transmitting frames to stationSTA4 of the basic service set BSS-B. Another shortcoming of thestructure of FIG. 1 is that, as there is no feedback required toindicate the correct reception of the broadcast/multicast frames, theconventional scheme of broadcast and multicast of data, audio or videoframes is unreliable.

To provide reliable unicast, broadcast and multicast transmission ofdata, audio or video frames, one approach is to decrease the probabilityof transmission collisions. Another approach is to provide feedback,such as acknowledgement, in response to receipt of data, audio or videoframes transmitted via broadcast or multicast.

FIG. 2 illustrates a TXOP information element according to oneembodiment. In an inventive reservation-based medium access scheme todecrease the probability of transmission collisions, a TXOP informationelement, which is a set of information, is used to reserve a TXOP timeinterval. It is believed that reliable transmission and deterministicdelay can be achieved by transmission of high-priority frames such asaudio and video frames during the reserved TXOP time interval. Thereservation of a TXOP time interval is made known by transmission, suchas broadcast or multicast, of the TXOP information element to stationsin a basic service set. In one embodiment, the access point of a basicservice set transmits a TXOP information element to a number of stationsin the basic service set to reserve a TXOP time interval fortransmission of data, audio or video frames. For example, the accesspoint may broadcast the TXOP information element to all stations in thebasic service set to reserve a time interval for transmission of data,audio or video frames to avoid intra-basic service set collisions duringthe transmission of the data, audio or video frames. Each station, uponbeing notified of the reserved TXOP time interval, will refrain fromtransmission during the reserved TXOP time interval. For instance, eachstation informed of the reserved TXOP time interval will set its networkallocation vector (NAV) to refrain from transmission during the reservedTXOP time interval in accordance with the IEEE 802.11 standards.

As shown in FIG. 2, a TXOP information element includes the followingfields: an active indication field, an activation start time field, aTXOP type field, a source address field, a destination address field, aservice interval field, an offset to service interval field, and aduration field. Alternatively, a TXOP information element may includefewer or more of the fields listed above. In one embodiment, a TXOPinformation element includes the information needed to reserve one TXOPtime interval for transmission of data, audio or video frames during oneservice interval. A service interval is a period of time sufficientlylong to include one or more TXOP time intervals for an access point totransmit data, audio, or video frames, whether by broadcast, multicast,and/or unicast. Alternatively, a TXOP information element may includethe information needed to reserve more than one TXOP time intervals.

In one embodiment, the active indication field is set to a first valueif and only if the TXOP is currently enabled, and set to a second valueif the TXOP is not currently enabled. The activation start time fieldindicates when the TXOP will be enabled if the active indication is setto the second value indicating the TXOP is not currently enabled. Insome embodiments, several transmissions of the TXOP information elementwith the active indication field set to the second value may be neededin order to ensure all stations in the basic service set have beennotified of an imminent TXOP. The TXOP type field indicates whether theTXOP is a one-stage TXOP or a two-stage TXOP. For example, when there isno overlapping basic service set detected, a TXOP will be of theone-stage type as will be described in more detail below. Conversely, ifat least one overlapping basic service set is present, the TXOP will beof the two-stage type as will be described in more detail below.

The duration field indicates the length of the reserved TXOP timeinterval. Under the reservation-based medium access scheme, it is duringthe reserved TXOP time interval that data, audio or video frames aretransmitted to avoid collisions. The source address field indicates theaddress of the access point for broadcast and multicast transmission inone embodiment. Alternatively, the source address field may indicate theowner of the reserved TXOP, which may be the access point. Thedestination address field indicates the address or addresses of theintended recipient or recipients of the video frames. For a broadcasttransmission, the destination address field indicates the addresses ofall the stations in the basic service set. For a multicast transmission,the destination address field indicates the addresses of those stationsin the BSS the multicast transmission is directed to. For a unicasttransmission, the destination address field indicates the address of thestation in the basic service set the unicast transmission is directedto. The service interval field indicates the length of the serviceinterval specific to the access point, and the length may differ fromaccess point to access point. The offset to service interval fieldindicates an offset between the beginning of transmission of the data,audio or video frames and the beginning of the respective serviceinterval, and the offset may be adjusted according to the traffic on thetransmission medium.

FIG. 3 illustrates the relationship between the transmission of TXOPinformation element and the reserved TXOP time intervals according toone embodiment. A TXOP information element is periodically broadcastedby the access point to the stations in a basic service set in a beaconto reserve one or more TXOP time intervals in a given service interval.In another embodiment, the TXOP information element is periodicallybroadcasted in an action frame such as a management frame to thestations in the basic service set. The periodic reservation of TXOP timeintervals is believed to result in reliable transmission of data, audioor video frames during the reserved TXOP time intervals anddeterministic delay.

When there are a number of service intervals between two adjacentbeacons broadcasted by the access point, the TXOP information element isused to reserve one or more TXOP time intervals during each of thenumber of the service intervals between two beacons. For example, asshown in FIG. 3, the TXOP information element broadcasted in beacon 1reserves the different TXOP time intervals TXOP1, TXOP2, and TXOP3. Asthere are two service intervals between beacon 1 and beacon 2, the TXOPinformation element broadcasted in beacon 1 reserves the TXOP timeintervals TXOP1, TXOP2, and TXOP3 for both of the service intervals. TheTXOP time intervals TXOP1, TXOP2, and TXOP3 may have varying durationdepending on the bandwidth requirements. If more data, audio or videoframes need to be transmitted after the two service intervals, anotherTXOP information element will be broadcasted in beacon 2 to reserve moreTXOP time intervals in the subsequent service intervals.

An access point reserves TXOP time intervals according to servicerequirements from the upper layers above the access point's MAC layer oraccording to service requirements from the stations associated with theaccess point. Scheduling of the TXOP time intervals depends on theimplementation.

FIG. 4 illustrates the time spots for transmission of a TXOP informationelement according to one embodiment. The access point should ensure thatall stations associated with it receive the TXOP information element.

To do so, in one embodiment the access point periodically broadcasts theTXOP information element in one or more beacons plus one or morebroadcast TXOP reservation action frames. In case one or more stationsin the basic service set are in power saving mode, the access point mayperiodically broadcast the reserved video TXOP information element inone or more DTIM beacons, as shown in FIG. 4, to ensure that allstations receive the reserved video TXOP information element. In suchcase, the broadcasted TXOP information element contains the necessaryinformation for reservation of one or more TXOP time intervals for theservice intervals between two adjacent DTIM beacons. Alternatively, theTXOP information element may be periodically broadcasted in beacons atone or more target beacon transmission times in addition to beingbroadcasted in DTIM beacons.

FIG. 5 illustrates collision avoidance for inter-basic service setunicast, broadcast or multicast transmissions according to oneembodiment. In one embodiment, two overlapping basic service sets BSS1and BSS2 exchange information related to their respective reserved TXOPfor broadcast or multicast transmission through stations in theirrespective basic service set. The exchange is through relayed actionmanagement frame. For example, if station 1 of the basic service setBSS1 is within the transmission range of the basic service set BSS2 andstation 2 of the basic service set BSS2 is within the transmission rangeof the basic service set BSS1, then information related to the broadcastor multicast TXOP reserved by the basic service sets BSS1 and BSS2 isexchanged through station 1 and station 2 using relayed actionmanagement frame.

In one embodiment, the EDCA protocol is used to contend for mediumaccess right during the reserved TXOP. Alternatively, a differentprotocol (e.g., HCCA) within the IEEE 802.11 standards is used toacquire medium access right.

In one embodiment, the respective access point of each or at least oneof the basic service sets BSS1 and BSS2 records the difference of timingsynchronization function (TSF) values between the respective basicservice set and the overlapping basic service set. Based on thedifference of the TSF values, the overlap between the reserved TXOP ofthe basic service set BSS1 and the reserved TXOP of the basic serviceset BSS2 can be calculated. As one of the basic service sets BSS1 andBSS2 selects a new TXOP for unicast, broadcast or multicast, the newlyselected TXOP should not be the same or overlap the reserved TXOP of theoverlapping basic service set in order to avoid collisions. For example,as shown in FIG. 5, when the basic service set BSS2 reserves a new TXOP,the new reserved TXOP is not the same as and does not overlap thereserved TXOP of the basic service set BSS1.

FIG. 6 illustrates collision avoidance for inter-basic service setbroadcast or multicast transmissions according to another embodiment.Two overlapping basic service sets BSS1 and BSS2 reserve overlappingTXOP at almost the same time as shown in FIG. 6. In one embodiment, arandom priority number is selected for each newly reserved TXOP. Thenewly reserved TXOP having the lower priority number will be released bythe respective basic service set, and a new TXOP is selected to replacethe released TXOP.

For example, as shown in FIG. 6, the basic service set BSS1 reservesTXOP1 while the basic service set BSS2 reserves TXOP2 that overlapTXOP1. A priority number is then randomly selected for each of TXOP1 andTXOP2 to determine which one of the newly reserved TXOP1 and TXOP2should be released by the respective basic service set BSS1 or BSS2. IfTXOP1 has a lower priority number than that of TXOP2, then TXOP1 will bereleased by the basic service set BSS1 as TXOP2 is kept by the basicservice set BSS2. Conversely, if TXOP2 has a lower priority number thanthat of TXOP1, TXOP2 will be released by the basic service set BSS2 asTXOP1 is kept by the basic service set BSS1.

FIG. 7A illustrates collisions during inter-basic service set unicast,broadcast or multicast transmissions. Two overlapping basic service setsBSS1 and BSS2 use different beacon intervals. For example, the basicservice set BSS1 has a beacon interval that is shorter than the beaconinterval of the overlapping basic service set BSS2, as shown in FIG. 7A,or vice versa. If each access point uses its respective beacon intervalto select the service interval, offset, and duration of its reservedTXOP, then after some number of beacon intervals the reserved TXOPs ofthe overlapping basic service sets BSS1 and BSS2 will eventually overlapwith each other. This is because the TBTT and the reserved TXOPs of theoverlapping basic services sets BSS1 and BSS2 slide at different speeds.

As shown in FIG. 7A, the combined length of two service intervals of thebasic service set BSS1 is roughly covered by the combined length ofthree service intervals of the basic service set BSS2. Even though thereserved TXOPs of the basic service set BSS2 in the first serviceinterval do not overlap with the reserved TXOPs in the first and secondservice intervals of the basic service set BSS1, the reserved TXOPs inthe second service interval of the basic service set BSS2 do overlapwith the reserved TXOPs in the second and third service intervals of thebasic service set BSS1.

FIG. 7B illustrates collision avoidance for inter-basic service setbroadcast or multicast transmissions according to yet anotherembodiment. In one embodiment, the beacon interval of the access pointis not used to select the service interval, offset, or duration of thereserved TXOP. In an embodiment, the default beacon interval is used toselect the service interval, offset, or duration of the reserved TXOP.The difference between the TBTT and the target default beacon transmittime (TDBTT) is included in the reserved TXOP information element. Inanother embodiment, when a reserved service interval of a basic serviceset covers its own TBTT, the respective access point extends thereserved service interval to compensate the beacon transmit time.

As shown in FIG. 7B, both basic service sets BSS1 and BSS2 use thedefault beacon interval, i.e., the duration between two adjacent TDBTTs,to select TXOPs, overlap between the reserved TXOPs of the basic servicesets BSS1 and BSS2 can be avoided. In particular, so long as the basicservice sets BSS1 and BSS2 reserve TXOPs that do not overlap with anyTXOP reserved by the overlapping basic service set BSS1 or BSS2, overlapbetween reserved TXOPs should be avoided because the TDBTT is the samefor both of the basic service sets BSS1 and BSS2.

FIG. 8A illustrates inter-basic service set interference and collisionavoidance when a station receives reserved TXOP information element froman overlapping basic service set's access point according to oneembodiment. As shown in FIG. 8A, the station STA4 of the basic serviceset BSS2 and the access point AP1 of the basic service set bSS1interfere each other, given the proximity between the two. The stationSTA4 receives reserved broadcast or multicast TXOP information elementfrom the access point AP1 through management frames. In one embodiment,during the reserved TXOP of the basic service set BSS1, the station STA4refrains from receiving management frames, data frames, or controlframes including acknowledgement (ACK) frames from the basic service setBSS2. Further, during the reserved TXOP of the basic service set BSS1,station STA4 refrains from transmitting unicast frames requiring ACK,although the station STA4 may transmit non-ACK frames. Moreover, theaccess point AP2 of the basic service set BSS2 does not broadcast framesor multicast frames if the multicast address includes the station STA4during the reserved TXOP of the basic service set BSS1. In anotherembodiment, the station STA4 relays the reserved broadcast or multicastTXOP information element received from the access point AP1 to itsassociated access point, AP2, to notify access point AP2 of the reservedTXOP of the basic service set BSS1. Such information may be relayedusing an action frame, for example.

FIG. 8B illustrates inter-basic service set interference and collisionavoidance when an access point receives reserved TXOP informationelement from an overlapping basic service set's station according to oneembodiment. As shown in FIG. 8B, the station STA3 of the basic serviceset BSS1 and the access point AP2 of the basic service set bSS2interfere each other, given the proximity between the two. The accesspoint AP2 receives reserved broadcast or multicast TXOP informationelement from the station STA3 through management frames. In oneembodiment, during the reserved TXOP of the basic service set BSS1, theaccess point AP2 refrains from transmitting management frames, dataframes, or control frames including ACK frames. Further, during thereserved TXOP of the basic service set BSS1, the access point AP2refrains from acknowledging received unicast frames, although it maystill receive non-ACK frames. In another embodiment, the station STA3forwards the broadcast or multicast reserved TXOP information receivedfrom access point AP1 to its access point, AP2, to notify access pointAP2 of the reserved TXOP of the basic service set BSS1.

FIG. 8C illustrates inter-basic service set interference and collisionavoidance when an access point receives reserved TXOP informationelement from an overlapping basic service set's access point accordingto one embodiment. As shown in FIG. 8C, access point AP1 of the basicservice set BSS1 and the access point AP2 of the basic service set bSS2interfere each other, given the proximity between the two. The accesspoint AP2 receives reserved broadcast or multicast TXOP informationelement from the access point AP1 through management frames. In oneembodiment, during the reserved TXOP of the basic service set BSS1, theaccess point AP2 does not receive management frames, data frames, orcontrol frames including ACK frames from its associated stations in thebasic service set BSS2. Further, during the reserved TXOP of the basicservice set BSS1, the access point AP2 transmits broadcast frames,multicast frames, and non-ACK frames. The access point AP2, however,refrains from transmitting unicast frames that require acknowledgementduring the reserved TXOP of the basic service set BSS1. To fully protectvideo transmission, in one embodiment, the station STA4 relays theperiodic broadcast or multicast reserved TXOP information elementreceived from the access point AP1 to its associated access point, AP2,to notify access point AP2 of the reserved TXOP of the basic service setBSS1.

FIG. 8D illustrates inter-basic service set interference and collisionavoidance when a station receives reserved TXOP information element froman overlapping basic service set's station according to one embodiment.As shown in FIG. 8D, the station STA3 of the basic service set BSS1 andthe station STA4 of the basic service set bSS2 interferes each other,given the proximity between the two. The station STA4 receives reservedbroadcast or multicast TXOP information element from the station STA3through management frames. In one embodiment, during the reserved TXOPof the basic service set BSS1, the station STA4 refrains fromtransmitting management frames, data frames, and control framesincluding ACK frames. Further, during the reserved TXOP of the basicservice set BSS1, the station STA4 refrains from acknowledging receivedunicast frames although it may still receive unicast no-ACK frames. Inone embodiment, during the reserved TXOP of the basic service set BSS1,the access point AP2 transmits broadcast or multicast frames and thestation STA4 receives broadcast or multicast frames. In anotherembodiment, the station STA3 relays the reserved broadcast or multicastTXOP information element received from the access point AP1 to thestation STA4, so that the station STA4 can relay such information to itsassociated access point, AP2, to notify access point AP2 of the reservedTXOP of the basic service set BSS1.

FIG. 9 illustrates a process 100 of avoiding collision of intra-basicservice set broadcast or multicast transmissions according to oneembodiment. At 110, stations in the basic service set are notified of areserved TXOP for broadcast or multicast transmission. At 120,transmission from at least one station in the basic service set isdeferred until after the reserved broadcast or multicast TXOP.

In one embodiment, the notification comprises broadcasting a reservedTXOP information element related to the reserved TXOP. In anotherembodiment, the reserved TXOP information element is broadcasted in oneor more beacon by the access point of the basic service set.Alternatively, the reserved TXOP information element is broadcasted bythe access point in one or more action frame. In one embodiment, thereserved TXOP information element includes one or more of the following:an active indication of whether or not the TXOP is currently enabled, anactivation start time to indicate a time when the TXOP will be enabledif the TXOP is not currently enabled, a TXOP type, a source address, adestination address, a service interval, an offset to the serviceinterval, and a duration of the TXOP.

As it is possible that one or more of the stations in the basic serviceset may be in a power saving mode, the access point may need tobroadcast the reserved TXOP information element at a time other thanduring the regular beacon broadcast to ensure all stations associatedwith the access point, i.e., all the stations in the basic service set,receive the reserved TXOP information element. Accordingly, in oneembodiment, the access point broadcasts the reserved TXOP informationelement using a DTIM beacon. In one embodiment, upon being notified ofthe reserved broadcast or multicast TXOP, a station in the basic serviceset sets a network allocation vector (NAV) to stop frame transmissionduring the reserved broadcast or multicast TXOP. In another embodiment,all stations in the basic service set will set the NAV to refrain fromtransmission during the reserved TXOP upon being notified of thereserved TXOP.

FIG. 10A illustrates a process 200A for avoiding collision ofinter-basic service set uncast, broadcast or multicast transmissionsaccording to one embodiment. At 210, a first unicast, broadcast ormulticast TXOP is reserved in a first basic service set. At 220, asecond unicast, broadcast or multicast TXOP is reserved in a secondbasic service set, where the reserved second TXOP overlaps the reservedfirst TXOP. At 230, the first and the second basic service sets exchangeinformation related to the respective reserved TXOP, namely the firstTXOP and the second TXOP. At 240, a difference of TSF values between thefirst and the second basic service sets is determined. At 250, whetheror not an overlap between the reserved first and second TXOPs exists isdetermined, based on the difference between the TSF values. At 260, inat least one of the first and the second basic service sets, mediumaccess right is contended using the EDCA protocol during the respectivereserved TXOP. At 270, the difference of the TSF values between thefirst and the second basic service sets is recorded at the respectiveaccess point of either or both of the first and the second basic servicesets to calculate the overlap between the reserved first and secondTXOPs.

FIG. 10B illustrates a process 200B for avoiding collision ofinter-basic service set unicast, broadcast or multicast transmissionsaccording to another embodiment. At 210, a first unicast, broadcast ormulticast TXOP is reserved in a first basic service set. At 220, asecond unicast, broadcast or multicast TXOP is reserved in a secondbasic service set, where the reserved second TXOP overlaps the reservedfirst TXOP. At 230, the first and the second basic service sets exchangeinformation related to the respective reserved TXOP, namely the firstTXOP and the second TXOP. At 240, a difference of TSF values between thefirst and the second basic service sets is determined. At 250, whetheror not an overlap between the reserved first and second TXOPs exists isdetermined, based on the difference between the TSF values. At 260, inat least one of the first and the second basic service sets, mediumaccess right is contended using the EDCA protocol during the respectivereserved TXOP. At 280, a new unicast, broadcast or multicast TXOP notoverlapping the reserved second TXOP of the second basic service set isselected by the first basic service set when the reserved first TXOP ofthe first basic service set overlaps the reserved second TXOP of thesecond basic service set. Alternatively, a new unicast, broadcast ormulticast TXOP not overlapping the reserved first TXOP of the firstbasic service set is selected by the second basic service set when thereserved second TXOP of the second basic service set overlaps thereserved first TXOP of the first basic service set.

In one embodiment, information related to the respective reserved TXOPbetween the first and the second basic service sets is exchanged usingrelayed action management frames. In another embodiment, the exchange isconsummated using relayed action management frames through stations ofthe first and the second basic service sets. For example, if station 1of the first basic service set is within the transmission range of thesecond basic service set and station 2 of the second basic service setis within the transmission range of the first basic service set, theninformation related to the reserved first and second TXOPs is exchangedthrough station 1 and station 2.

FIG. 11 illustrates a process 300 for avoiding collision of inter-basicservice set unicast, broadcast or multicast transmissions according toanother embodiment. At 310, a first TXOP is reserved in a first basicservice set. At 320, a second TXOP is reserved in a second basic serviceset, where the second TXOP overlaps the first TXOP reserved by the firstbasic service set. At 330, a respective priority number is randomlyselected for each of the first and the second basic service sets. Thisprovides one of the first and the second basic service sets with ahigher priority number and the other with a lower priority number. At340, the basic service set having the lower priority number releases thereserved TXOP. At 350, the basic service set having the lower prioritynumber, after releasing the respective TXOP, selects a new TXOP thatdoes not overlap the TXOP reserved in the basic service set having thehigher priority number. Thus, it is believed the process 300 may break atie between two overlapping basic service sets when each basic serviceset reserves a TXOP that overlaps the TXOP of the other basic serviceset.

FIG. 12A illustrates a process 400A for avoiding collision ofinter-basic service set unicast, broadcast or multicast transmissionsaccording to still another embodiment. At 410, an overlap between atransmission range of a first basic service set and a transmission rangeof a second basic service set is detected. At 420, in reserving a TXOPfor unicast, broadcast or multicast transmission in each of the firstand the second basic service sets, at least one of a service interval,an offset, and duration is respectively selected based on a defaultbeacon interval. In one embodiment, the first basic service set has afirst beacon interval and the second basic service set has a secondbeacon interval that is different from the first beacon interval. At430, a difference between a first beacon interval of the first basicservice set and the default beacon interval is included in a reservedTXOP information element that is broadcasted to stations in the firstbasic service set. Alternatively, a difference between a second beaconinterval of the second basic service set and the default beacon intervalis included in a reserved TXOP information element that is broadcastedto stations in the second basic service set.

FIG. 12B illustrates a process 400B for avoiding collision ofinter-basic service set unicast, broadcast or multicast transmissionsaccording to yet another embodiment. At 410, an overlap between atransmission range of a first basic service set and a transmission rangeof a second basic service set is detected. At 420, in reserving a TXOPfor unicast, broadcast or multicast transmission in each of the firstand the second basic service sets, at least one of a service interval,an offset, and duration is respectively selected based on a defaultbeacon interval. In one embodiment, the first basic service set has afirst beacon interval and the second basic service set has a secondbeacon interval that is different from the first beacon interval. At440, a reserved service interval is adjusted if the reserved serviceinterval covers a first beacon interval of the first basic service setbefore the adjusting. Alternatively, a reserved service interval isadjusted if the reserved service interval covers a second beaconinterval of the second basic service set before the adjusting. Forexample, the access point of the first basic service set may extend thereserved service interval to compensate the beacon transmit time whenthe reserved service interval covers the TBTT of the first basic serviceset. In one embodiment, upon detecting an overlap between thetransmission ranges of the first and the second basic service sets, thefirst basic service set reserves a first TXOP based on the defaultbeacon interval and the second basic service set reserves a second TXOPnot overlapping the first TXOP based on the default beacon interval.

FIG. 13 illustrates a wireless communication device 501 in a WLANcommunication system 500 according to one embodiment. The wirelesscommunication device 501 includes a transmitter 510, a receiver 520, anda circuit 530. The control circuit 530 includes communication logic 540.The communication logic 540 is configured to notify other wirelesscommunication devices in the system 500 of a reserved TXOP for unicast,broadcast or multicast transmission. The wireless communication device501 is also configured to cause the transmitter 510 of the wirelesscommunication device 501 to cease transmission during a TXOP reserved byanother wireless communication device.

The system 500 is a basic service set that includes the wirelesscommunication device 501 and other wireless communication device 502,503 and 504. The wireless communication devices 501, 502, 503 and 504communicate with each other via the transmission medium 505 wirelessly,for example, according to the IEEE 802.11 standards, including theprotocols defined in the IEEE 802.11e amendment. In one embodiment, oneor more of the wireless communication devices 502, 503, and 504 arestructurally and/or functionally the same as the wireless communicationdevices 501. In another embodiment, one or more of the wirelesscommunication devices 502, 503, and 504 are structurally and/orfunctionally different from the wireless communication devices 501. Itshould be understood that, although four wireless communication devices501-504 illustrated in the system 500, there may be more or fewerwireless communication devices in the system 500 in other embodiments.

In one embodiment, the communication logic 540 is configured to causethe wireless communication device 501 to exchange information related toa respective reserved TXOP with another wireless communication device502, 503, or 504 in a second basic service set to determine a differenceof TSF values between the first and the second basic service sets. Basedon the difference of TSF values, the communication logic 540 determineswhether or not there is an overlapping reserved TXOP between the firstand the second basic service sets. In one embodiment, the communicationlogic 540 contends for medium access right using the EDCA protocolduring the reserved TXOP.

In another embodiment, the communication logic 540 randomly selects apriority number if the TXOP reserved by the communication logic 540overlaps a reserved TXOP of a second basic service set. Thecommunication logic 540 causes the wireless communication device 501 torelease the reserved TXOP if the randomly selected priority number islower than a randomly selected priority number for the second basicservice set, and to select a new TXOP that does not overlap the reservedTXOP of the second basic service set. In yet another embodiment, thecommunication logic 540 uses the default beacon interval to select atleast one of a service interval, an offset, and duration in reserving aTXOP for broadcast or multicast transmission.

Thus, embodiments of an inventive scheme to avoid collisions of inter-and intra-basic service set broadcast or multicast transmissions havebeen disclosed. The implementation scheme according to the variousembodiments disclosed herein provides several advantages over theconventional implementation of transmission of video frames. Forinstance, to avoid intra-basic service set collisions, stations in abasic service set are notified of a reserved TXOP for broadcast ormulticast transmission so that the stations can defer transmission untilafter the reserved broadcast or multicast TXOP. There is no physicallayer modification is necessary to implement the proposed scheme, yetrobust broadcast/multicast transmission of video frames may be achieved.

The above description of illustrated embodiments, including what isdescribed in the Abstract, is not intended to be exhaustive or to limitthe embodiments to the precise forms disclosed. Although specificembodiments of and examples are described herein for illustrativepurposes, various equivalent modifications can be made without departingfrom the spirit and scope of the disclosure, as will be recognized bythose skilled in the relevant art. The teachings provided herein of thevarious embodiments can be applied to other context, not necessarily theexemplary context of transmission of video frames generally describedabove. It will be understood by those skilled in the art that, althoughthe embodiments described above and shown in the figures are generallydirected to the context of transmission of video frames, applicationsrelated to transmission of high-priority frames other than video frames,for example, may also benefit from the concepts described herein.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

We claim:
 1. A method of avoiding collision of intra-basic service setunicast, broadcast or multicast transmissions, the method comprising:detecting traffic on a transmission medium of the intra-basic serviceset; adjusting a reserved time interval with a service interval offsetbased on the detected traffic; notifying, via a central reservationwireless communication device, stations in the basic service set of atransmit opportunity for the reserved time interval for a unicast,broadcast or multicast transmission, wherein the notifying includesbroadcasting at least one of an active indication to indicate thetransmit opportunity is currently enabled and an activation start timeto indicate a time when the transmit opportunity will be enabled if thetransmit opportunity is not currently enabled; and deferringtransmission from at least one station in the basic service set untilafter the reserved time interval.
 2. The method of claim 1 whereinnotifying the stations in the basic service set of the transmitopportunity comprises broadcasting an information element related to thetransmit opportunity in at least one beacon or action frame to thestations in the basic service set.
 3. The method of claim 1 whereinnotifying the stations in the basic service set of the transmitopportunity comprises broadcasting the transmit opportunity using adelivery traffic indication message (DTIM) beacon to stations in thebasic service set.
 4. The method of claim 1 wherein deferringtransmission from at least one station in the basic service set untilafter the reserved time interval comprises setting a network allocationvector (NAV) in the at least one station in the basic service set tostop frame transmission during the reserved time interval.
 5. An accesspoint of an intra-basic service set, comprising: a transmitter; areceiver; a control circuit coupled to the transmitter and the receiver,the control circuit arranged to reserve a unicast, broadcast ormulticast transmit opportunity (TXOP) in the intra-basic service set,the control circuit further arranged to format the unicast, broadcast,or multicast TXOP to direct the transmitter to transmit an informationelement having fields, the fields including: an active indication fieldconfigurable to indicate whether or not the TXOP is currently enabled;an activation start time field configurable to indicate a time when theTXOP will be enabled if the TXOP is not currently enabled; a TXOP typefield; a source address field; a destination address field; a serviceinterval field; an offset to the service interval field; and a durationof the TXOP field.
 6. The access point of claim 5 wherein the controlcircuit logic to reserve a unicast, broadcast or multicast transmitopportunity (TXOP) is configured to direct the transmitter to transmitthe information element during a beacon broadcast and at a time otherthan during the beacon broadcast.
 7. The access point of claim 5 whereinthe control circuit logic to reserve a unicast, broadcast or multicastTXOP is configured to set the service interval field to indicate atleast one TXOP time interval is being reserved.
 8. The access point ofclaim 5 wherein the control circuit logic to reserve a unicast,broadcast or multicast TXOP is configured to set the service intervalfield to indicate one TXOP time interval is being reserved in a firsttransmission and configured to set the service interval to indicate atleast two TXOP time intervals are being reserved in a secondtransmission.
 9. The access point of claim 5 wherein the control circuitlogic to reserve a unicast, broadcast or multicast TXOP is configured toset the service interval field based on a specific characteristic of theaccess point.
 10. The access point of claim 5 wherein the controlcircuit logic to reserve a unicast, broadcast or multicast TXOP isconfigured to adjust the offset to service interval field based ontraffic on a transmission medium, the traffic indicated by informationreceived via the receiver.
 11. The access point of claim 5 wherein thecontrol circuit logic to reserve a unicast, broadcast or multicast TXOPis configured to set the active indication field to a first value if andonly if the unicast, broadcast or multicast TXOP is currently enabledand configured to set the active indication field to a second value ifthe unicast, broadcast or multicast TXOP is not currently enabled. 12.The access point of claim 11 wherein the control circuit logic toreserve a unicast, broadcast or multicast TXOP is configured to set theactivation start time field when the TXOP will be enabled if the activeindication field is set to the second value.
 13. The access point ofclaim 12 wherein the control circuit logic to reserve a unicast,broadcast or multicast TXOP is configured to direct the transmitter totransmit the TXOP information element a plurality of times when activeindication field is set to the second value.
 14. The access point ofclaim 5 wherein the control circuit logic to reserve a unicast,broadcast or multicast TXOP is configured to detect an overlapping basicservice set using information received via the receiver and configuredto set the TXOP type field to a first value or a second value based onthe detection.
 15. A system including two or more stations and an accesspoint, the two or more stations and the access point formed into anintra-basic service set, the access point configured as a centralreservation wireless communication device, the access point, comprising:a transmitter; a receiver; and a control circuit coupled to thetransmitter and the receiver, the control circuit configured to reservea unicast, broadcast or multicast transmit opportunity (TXOP) in theintra-basic service set, the control circuit further configured todirect the transmission of an information element indicating whether ornot the TXOP is currently enabled and an activation start time toindicate a time when the TXOP will be enabled if the TXOP is notcurrently enabled, and the control circuit further configured to detecttraffic on a transmission medium of the intra-basic service set andadjust a service interval offset based on the detected traffic.
 16. Thesystem of claim 15 wherein the control circuit is further configured todirect a plurality of transmissions of the information element, at leastone transmission during a beacon broadcast and at least one transmissionduring an action frame.
 17. The system of claim 15 wherein the controlcircuit is further configured to reserve at least two TXOP timeintervals.
 18. A stem, comprising: a first station; a second station;and an access point configured as a central reservation wirelesscommunication device in an intra-basic service set formed with the firstand second stations, the access point including a transmitter, areceiver, and a control circuit coupled to the transmitter and thereceiver, the control circuit configured to: reserve a unicast,broadcast or multicast opportunity (TXOP) in the intra-basic serviceset; direct the transmission of an information element indicatingwhether or not the TXOP is currently enabled and if the TXOP is notcurrently enabled, the information element indicating an activationstart time when the TXOP will be enabled; detect an overlapping basicservice set; and set a TXOP type field based on the detection.